Stars are born in enormous clouds of gas and dust like the Orion Nebula, seen here in a mosaic of images from the Hubble Space Telescope. In such nebulas, cold gas collapses into denser clumps over millions of years. Eventually, temperature and pressure within the clumps rise high enough to ignite nuclear fusion—and a star is born. Scientists think the shock waves from supernovas may serve as the trigger for star formation, jostling the gas clouds and causing them to collapse.

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